Open the front door.
Pull the blue plastic lever on the right towards you.
This opens a little window on the right hand side of the printer towards the rear.
Look inside, pull out the blue plastic rod. It has a sponge on the bottom.
Push it in. Pull it out. Ten times.
This will clean any spillage from the laser lense.
If it is on all 4 print colours this is the most likley cause